Percent error formula
Percent error = |Measured − True| ÷ |True| × 100
Example: measuring gravity as 9.8 m/s² against the accepted 9.81 m/s² gives |9.8 − 9.81| ÷ 9.81 × 100 = 0.102%.
Percent error uses the absolute value, so it's always positive; some science classes keep the sign to show whether the measurement was high or low, shown here as the direction.
Related measures
- Absolute error is the plain difference, in the original units.
- Relative error is the same as percent error without multiplying by 100.
- Percent difference compares two measurements when neither is the true value: |A − B| ÷ ((A + B) ÷ 2) × 100.
For changes over time, use the percent change calculator.
Frequently asked questions
How do you calculate percent error?
Subtract the true value from the measured value, take the absolute value, divide by the true value and multiply by 100.
What is a good percent error?
It depends on the experiment. In many school labs, under 5% is considered good and under 1% excellent.
Can percent error be negative?
With the usual formula, no, because it uses absolute value. If the sign is kept, a negative result means the measurement was too low.
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